1. Field of the Invention
The present invention relates to an optical information recording medium provided with an optically detectable information recording layer, the producing method thereof and a method of recording/erasing/reproducing information.
2. Description of the Prior Art
A recording material thin film layer comprising a metal thin film and an organic thin film is formed on a disc-shaped or a card-shaped substrate. A high energy beam focused on a micro light spot having a submicron order diameter is irradiated onto the recording material layer, thereby a local variation is generated on recording material layer. Thereby, such a technique that an information signal is stored is already well known. More specifically, when an optical magnetic material thin film and a phase change material thin film are used for a recording layer, it is easy to rewrite the signal. Accordingly, this technique has been actively studied and developed. For example, in case of the optical magnetic recording medium, a difference of a rotating angle on a polarized surface of a reflected light generated due to the difference of a magnetization state is used as the record. Furthermore, in case of the phase change recording medium, an amount of a reflected light relative to a light having a specific wavelength in a crystalline state is different from that in an amorphous state, thereby the difference is used as the record. A laser output is modulated between a record level having a relatively higher power and an erasure level having a relatively lower power, and the modulated output is only irradiated on a recording medium. Thereby similarly to a magnetic disk, there is such a characteristic that the record erasure and the record of a new signal can be simultaneously performed (it is possible to overwrite the record). The information signal can be rewritten for a short time.
Usually, the optical magnetic recording medium and the phase change recording medium comprise, for example, a multi-layer film shown in FIG. 1. That is, on a substrate 1 comprising a resin plate of a polycarbonate and PMMA (polymethyl-methacrylate), a glass plate, or the like, usually, a recording layer 3 having an optical absorption comprising the phase change material and the optical magnetic material inserted between protective layers 2 and 4 comprising a dielectric material is formed. Furthermore, a metallic reflecting layer 5 comprising an alloy of Au and Al for increasing an optical absorption efficiency on the recording layer 3 and for acting as a thermal diffusion layer is formed on the protective layer 4. These layers are sequentially laminated by a sputtering method, a vacuum deposition method, or the like. Furthermore, an overcoat layer 6 is formed on an uppermost layer in such a manner that a scratch and dusts are not attached to these layers. Usually, a laser beam is incident from a side of the substrate 1. In many cases, a front surface of the substrate 1 is provided with a concave-convex groove track or a concave-convex pit sequence as guide means for guiding the laser beam to a predetermined position on the disk.
A function of each layer and a concrete example of materials forming each layer are as follows.
In case of the recording layer 3, when the phase change material is used, chalcogenite thin film whose base comprises Te and Se, for example, a Ge—Sb—Te alloy thin film, a Ge—Sb—Te—Se alloy thin film, an In—Sb—Te alloy thin film, an Ag—In—Sb—Te alloy thin film, an In—Se alloy thin film, and the like are reported. In the medium using such phase change materials, the laser beam is irradiated, thereby the signal is recorded and reproduced. As already described, while the power of the laser beam is being modulated at a strong level and a weak level, the laser beam is irradiated onto a revolving recording medium. A portion irradiated with the strong power is locally melt in an instant, thenceforth, the portion is quenched. Thereby the portion is amorphized, and the signal is recorded. Furthermore, at the portion irradiated with a relatively weak power, the amorphous-state portion is annealed, thereby the portion is crystallized, and the recorded signal is erased. In order to reproduce the signal, the power of the laser beam is reduced enough in such a manner that the recording film is not changed, and the laser beam is irradiated. At this time, a strength of the reflected light is detected, and whether the portion irradiated with the laser beam is in the crystalline state or the amorphous state is judged, thereby the signal is reproduced.
The functions of the protective layers 2 and 4 comprising a dielectric material are, for example, as follows:                1) the recording layer is protected from an external mechanical damage;        2) a thermal damage such as a roughness on the surface of the substrate, a break of the recording layer and an evaporation, etc. occurred due to repeatedly rewriting the signal are reduced, thereby a repetition of rewriting the signal can be increased;        3) an interference effect of a multipath reflection is used so that an optical change can be enhanced;        4) an influence from an outside air is intercepted so that a chemical change can be prevented.        
As the material comprising the protective layer for satisfying the above objects, heretofore, an oxide such as SiO2, Al2O3 or the like, a nitride such as Si3N4, AlN or the like, an acid nitride such as Si—O—N or the like (for example, disclosed in Japanese Patent Application Laid-open No. 3-104038), a sulfide such as ZnS or the like, a carbide such as SiC or the like, or a mixed material such as ZnS—SiO2 or the like (disclosed in Japanese Patent Application Laid-open No. 63-103453) is proposed, and one part of them is practically used.
Two layers are provided to the protective layer, thereby the characteristic thereof can be enhanced. The example of the phase change recording medium is disclosed in Japanese Patent Application Laid-open No. 5-217211. That is, the dielectric layer comprising the nitride (SiN, AlN) and the carbide (SiC) is used at the side contacted to the optical recording layer as the protective layer of the optical recording layer including Ag, and ZnS or a compound including ZnS is used as the outer layer of the dielectric layer. The above SiN, SiC, AlN layer is used, thereby a combination of Ag included in the recording layer and S in the protective layer is prevented. As disclosed in Japanese Patent Application Laid-open No. 5-217211, a film thickness of the SiN, AlN, SiC layer is ranging from 5 nm to 50 nm. Furthermore, as disclosed in Japanese Patent Application Laid-open No. 6-195747, the protective layer has two layers inserted between the recording layer and the substrate, where one layer contacted to the recording layer comprises Si3N4 layer and the other layer contacted to the substrate comprises ZnS—SiO2 layer, thereby two dielectric layers are formed. The Si3N4 layer facilitates a crystallization of the phase change material layer.
The example of the optical magnetic recording medium is disclosed in Japanese Patent Application Laid-open No. 4-219650. Here, the dielectric layer contacted to the substrate has two layers, and one layer contacted to the substrate is a silicon oxide film, thereby an addhesiveness of the substrate and the dielectric layer is enhanced. Furthermore, the other layer contacted to the recording layer comprises the compound of the carbide and the nitride, thereby it is possible to prevent a corrosion of the magnetic recording layer occurred due to that oxygen from the silicon oxide layer and water passing through the substrate are penetrated into the recording layer. As disclosed in Japanese Patent Application Laid-open No. 4-219650, preferably, the nitride comprises Sn—N, In—N, Zr—N, Cr—N, Al—N, Si—N, Ta—N, V—N, Nb—N, Mo—N and W—N, and the film thickness thereof is ranging from 10 nm to 20 nm. Furthermore, as disclosed in Japanese Patent Application Laid-open No. 4-321948, in the same view of Japanese Patent Application Laid-open No. 4-219650, the dielectric layer contacted to the substrate has two layers. Here, one layer near the substrate comprises at least one kind of oxides selected from a group of Si, Zr, Y, Mg, Ti, Ta, Ca and Al, thereby the adhesiveness of the dielectric layer and the substrate is enhanced. Furthermore, the other layer contacted to the optical magnetic recording film comprises the nitride layer comprising at least one kind of nitrides selected from the group of Si, Zr, Y, Mg, Ti, Ta, Ca and Al, thereby it is suppressed that oxygen and water from the oxide layer are penetrated and diffused into the recording film layer. As disclosed in Japanese Patent Application Laid-open No. 4-321948, the film thickness of the nitride layer is ranging from 50 nm to 200 nm.
It is known that the protective layer is formed of a complex material comprising different substances so as to provide the film with good quality. For example, Japanese Laid-Open Patent Publication (Tokkai-Sho) No. 63-50931 discloses an example including a protective layer with good quality such as excellent adhesiveness with a substrate by adding at least either one of aluminum oxide and silicon oxide to a complex dielectric of aluminum nitride and silicon nitride and by defining the refractive index.
Japanese Laid-Open Patent Publication (Tokkai-Hei) No.2-105351 discloses an example including a protective layer having excellent adhesiveness with a substrate and excellent ductility formed of a complex dielectric comprising a nitride of silicon and indium.
Furthermore, Japanese Laid-Open Patent Publication (Tokkai-Hei) Nos. 2-265051, 2-265052 disclose examples including a protective layer formed of Si, N and an element having a smaller specific electric resistance than Si, so that the protective layer is hardly cracked and protects the recording layer sufficiently.
In general, the reflecting layer 5 comprises a metal such as Au, Al, Cr, Ni, Ag or the like and the alloy based upon these metals, and the reflecting layer 5 is disposed in such a manner that a radiation effect and an effective optical absorption of the recording thin film can be obtained.
As described above, in general, a sputtering method, a vacuum deposition method or the like is used as the method of preparing the recording medium. Furthermore, a reactive sputtering method is used so that the nitride can be contained in the thin film.
For example, as the method of producing an ablation type write once medium, such a method that N is contained in the Te-containing recording layer by the reactive sputtering is disclosed in Japanese Patent Application Laid-open No. 63-151486. As disclosed in Japanese Patent Application Laid-open No. 63-151486, a mixed gas of Ar and nitride is discharged relative to a telluric selenium alloy target. After the recording film containing tellurium, selenium and nitride on the substrate is formed by the reactive sputtering method, a nitrogen gas is introduced, and a nitrogen plasma is generated, thereby a surface layer having a high nitrogen density than an inside of the recording layer is formed. The surface of the recording film is nitrided, thereby a weather-proofness and a sensitivity are enhanced, and further a power tolerance is increased. The nitrogen density of the nitride layer is ranging from 2% to 10%, preferably, it is ranging from 2% to 20%. Preferably, the thickness of the surface layer is ranging about from 1 nm to 10 nm.
Furthermore, the example of the ablation type recording material is also disclosed in Japanese Patent Application Laid-open No.63-63153. The target comprising a material containing Te and Se is sputtered in a nitriding-oxide gas, a nitric dioxide gas or a gas containing a nitric dioxide, thereby the layer containing Te, Se and N is formed in the recording layer.
Furthermore, as disclosed in Japanese Patent Application Laid-open No. 4-78032, the surface of a metallic target is sputtered by Ar gas, and on the surface of the metallic element substrate is reacted with oxygen gas or nitrogen gas, thereby a metallic oxide film or a metallic nitride film is formed.
Furthermore, although omitted in the drawings, in order that an oxidization of the optical information recording medium or an attachment of dusts, etc. is prevented, such a structure that the overcoat layer is placed on the metallic reflecting layer 5, such a structure that an ultraviolet curing resin is used as an adhesive so that a dummy substrate is laminated, or the like is proposed.
However, it is known that the phase change optical recording medium has the following problems. That is, when the thin film comprising a material whose base is Te, Se, etc. containing Ge, Sb, In, etc. is used as the recording layer, and further the thin film comprising an oxide system material including such as SiO2 representatively, the thin film comprising a sulfide system material including such as ZnS representatively, or the thin film comprising a mixture system material including ZnS—SiO2 between the above two thin films is used as the protective layer, a laser irradiation is carried out. Thereby, the record and erasure of the information signal, and the like are repeated, thereby optical characteristics of the recording layer and the protective layer (such as a reflectivity, an absorptivity and the like) are changed. Accordingly, such a phenomenon that a recording characteristic or an erasure characteristic is changed. That is, the signal is repeatedly rewritten, thereby the reflectance of the medium is reduced, an amplitude of the signal is gradually reduced, or a jitter value at a marked position of a recording mark becomes larger, thereby an error rate of the recording signal becomes higher. Therefore, when the signal is reproduced, a readout error is occurred. Accordingly, there is such a problem that a possible times of rewriting is limited.
Principal causes of this change are as follows. That is, one cause is that an S component and an O component are diffuse and penetrate from the protective layer to the recording layer, on the contrary, the component such as Te, Se, etc. having a relatively high vapor pressure among the components of which the recording layer consists of diffuse from the recording layer to the protective layer. Furthermore, another cause is that one part of the protective layer material is chemically reacted with the recording layer. It is considered that the change is occurred due to either of the above causes, or a combination of the above causes.
In fact, according to an experiment by inventors, etc., in the optical disk applying a Ge—Sb—Te recording film and a ZnS—SiO2 protective layer, the S component is discharged from the protective layer due to the laser irradiation. Consequently, it is observed that an S atom is penetrated from the protective layer to the recording layer. Furthermore, it is also observed that the other Zn atom, Si atom and O atom are also diffused to the recording layer. In this case, although it is assumed that other elements are easy to move by a separation of the S atom, the mechanism thereof is not clear.
The phenomenon and the mechanism have not been clearly reported. In case that the nitride thin film including Si3N4 and AlN is used as the protective layer, the S component is not discharged, differently from the above example. On the other hand, an adherence to the recording layer of such a nitride is lower than that of ZnS—SiO2 film. For example, under an environment having a high temperature and a high humidity, there is another problem that a peeling is occurred. That is, when oxide such as SiO2, Ta2O5, Al2O3 and the like and nitride such as Sl3N1 AlN and the like are used as a dielectric material, since such a dielectric, material is less adhesive to a phase change type recording material, for example, under the high-temperature and high-humidity environment, the peeling and crack are occurred. Thereby, there is further problem that oxide such as SiO2, Ta2O5, Al2O3 and the like and nitride such as Si3N4, AlN and the like cannot be applied to a dielectric layer material.
A deterioration mechanism is summarized. In the first place, the more the times of repeating is increased, the more the above atom diffusion and chemical reaction are proceeded. Consequently, a composition in the recording layer is largely varied, thereby variations of the reflectance, the absorption and the like, and the variation of the recording characteristic (an amorphization sensitivity) and the erasure characteristic (a crystallization sensitivity and a crystallization rate) are actualized. It is supposed that in the protective layer, accompanied by the change of the optical characteristic, the composition changed, thereby such a change that a mechanical strength is reduced occurs. It can be considered a ZnS—SiO2 film widely applied as an excellent protective layer has a high adhesiveness between the protective layer and the recording layer and this results from the atomic diffusion. Furthermore, it is also considered that such a protective layer substantially contains a limit of the repeating times.
Relating to a material containing Ag and S, that is, the elements which are easy to chemically react, the method of suppressing the reaction is disclosed in Japanese Patent Application Laid-open No. 5-217211. However, the following view is not disclosed in the above prior art. That is, relative to the phase change recording medium such as Ge—Sb—Te system, In—Sb—Te system and the like being developed for an application as the most possible material system, in order to enhance the cycle performance thereof, the layer comprising the material such as nitride, nitriding-oxide, etc. is formed between a dielectric protective layer and a phase change recording layer. The formed layer acts as a barrier layer for preventing an interdiffusion and the chemical reaction between the recording layer and the protective layer. Furthermore, more specifically, Ge—N or Ge—N—O is superior as the dielectric protective layer material which does not substantially have the above problem. This material has also an excellent performance as the barrier layer. This is not also disclosed in the prior art.